1. Field of the Invention
The present invention relates to a mold clamping apparatus for injection molding machines, in which a tie bar for movably supporting a movable plate is provided with a clamping cylinder.
2. Description of the Relevant Art
A mold clamping apparatus for injection molding machines is known which has: a stationary plate; a plurality of parallel tie bars of which one end portion is secured to the stationary plate; a plurality of different cylinders accommodating a piston section set at the middle of each tie bar respectively, and are movable along the tie bars; a movable plate integrated with each cylinder; and a hydraulic circuit for selectively feeding pressurized oil to the oil chamber of each cylinder; whereby various mold opening/closing speeds or various mold opening/closing forces can be selected.
For example, the official gazette of Japanese patent publication No. 53-6668 discloses a mold clamping apparatus having a cylinder in which the diameter of the one side of the tie bar differs from the diameter of the other side of the tie bar, wherein a piston section set at the middle of the tie bar serves as a boundary between the two sides, so that the pressure area of each piston section differs.
The official gazette of Japanese utility model publication No. 62-22348 discloses a mold clamping apparatus in which the diameter of a tie bar and that of a piston section set at the middle of the tie bar are made different from each other in different cylinders, so that the pressure area of each piston section differs. For this mold clamping apparatus, two of four cylinders are made as rapid traverse cylinders for performing high-speed movement, and the other two cylinders are made as clamping cylinders for performing high-power clamping, and are made to operate independently. Therefore, the diameter of each tie bar is set so as to be suitable for the particular cylinder function.
However, the existing mold clamping apparatus requires various types of tie bars because each pressure area is set by varying the diameter of each tie bar. Therefore, increased man-hours for machining of tie bars and assembling them with piston sections and the like leads to greatly increased manufacturing cost.
Moreover, because tie bars with different diameters may become mixed, problems may arise with the balance, uniformity, and stability of the clamping force throughout the whole mold clamping control. For example, tie bars with small diameter are easily distorted, and may degrade the quality of products and the accuracy of control.